BMI calculator

The formula, with two examples

BMI is weight in kilograms divided by height in meters, squared: BMI = kg ÷ m².

For someone 175 cm tall weighing 70 kg: 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.86, shown as 22.9.

In pounds and inches the same index needs a factor of 703: BMI = 703 × lb ÷ in². At 5 ft 9 in (69 inches) and 155 lb: 69 × 69 = 4,761, 703 × 155 = 108,965, and 108,965 ÷ 4,761 = 22.89, again 22.9 once rounded to one decimal, the way BMI is usually quoted. Octoolo converts feet, inches and pounds to metric with the exact factors and uses the first formula, so both routes agree.

Because height is squared, it matters more than weight. The same 70 kg gives 23.1 at 174 cm and 22.6 at 176 cm, so measure height without shoes, standing straight against a wall.

Adult BMI categories

These are the standard adult bands used by the World Health Organization and the CDC. BMI was designed to describe whole populations, and doctors use it as a quick first check alongside other measures. Obesity is often split further into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 and above); Octoolo shows all of them as Obesity.

Turned around, the bands give a weight range for a height. At 175 cm, a BMI of 18.5 to 24.9 is about 56.7 to 76.3 kg: multiply each limit by the height in meters squared, 3.0625.

Some health services use lower cut-offs for people of South Asian, Chinese and other Asian family backgrounds, because health risks start at lower BMIs in those groups. The UK's NICE, for example, uses 23 and 27.5 in place of 25 and 30.

What BMI cannot see

• Muscle. BMI counts all weight the same. A muscular athlete can read as overweight or even obese with little body fat.
• Age and build. Older adults often lose muscle, so a normal BMI can hide more fat than it suggests. Frame size and body shape are not measured at all.
• Where fat sits. Fat around the waist carries more risk than fat elsewhere, and BMI cannot tell them apart; doctors often add a waist measurement.
• Children and teenagers. For ages 2 to 19, BMI is compared with growth charts for age and sex, as a percentile. The adult bands do not apply, so the category Octoolo shows is for adults only.
• Pregnancy. Weight gain is expected in pregnancy, so BMI does not describe it.

Octoolo says as much under the result: BMI is a rough guide for adults and does not account for muscle, age or build. It is not medical advice; a doctor can put the number in context.

What one point of BMI means in kilograms

Because the formula divides by height squared, the same weight change moves BMI more for a shorter person. At 175 cm, one point of BMI is 3.06 kg, about 6.75 lb: the height in meters, squared. At 5 ft 4 in a point is about 5.8 lb, and at 6 ft 2 in about 7.8 lb. In pounds the rule is height in inches, squared, divided by 703.

That makes goals easier to picture. Someone 175 cm tall with a BMI of 27 who wants to reach 24.9 has 2.1 points to go, roughly 6.4 kg.

Weight also moves during the day with meals and fluids, so weigh at the same time of day on the same scale, and watch the trend over weeks rather than any single reading.